it depends on how your sump is set up. My last sump was set up Skimmer -> fuge/chaeto -> Return section with a bubble trap before the return. I didn't see much point in doing over-under-over, so I just did under-over. What I realized later was that water would fall over the first baffle from the skimmer section, then tend to continue down the baffle and across the floor of the middle section, under the first baffle of the bubble trap and into the return section, meaning the water in the middle section was relatively stagnant because it didn't mix much.